There is an ongoing need in the electronics industry for increasingly higher circuit densities in microelectronic devices made using lithographic techniques. One method of increasing the number of components per integrated circuit (“chip”) is to decrease the minimum feature size on the chip, which requires higher lithographic resolution. This decrease in feature size has been accomplished over the past twenty years by reducing the wavelength of the imaging radiation from the visible (436 nm) down through the ultraviolet (365 nm) to the deep ultraviolet (DUV; 248 nm). Development of commercial lithographic processes using ultra-deep ultraviolet radiation, particularly at 193 nm or 157 nm, is now of increasing interest. See, for example, Allen et al. (1995), “Resolution and Etch Resistance of a Family of 193 nm Positive Resists,” J. Photopolym. Sci. and Tech. 8(4):623-636, and Abe et al. (1995), “Study of ArF Resist Material in Terms of Transparency and Dry Etch Resistance,” J. Photopolym. Sci. and Tech. 8(4):637-642.
Attempts have been made to develop 157 nm resists, for example, by using heavily fluorinated materials such as polytetrafluoroethylene (e.g., Teflon AF®; see Endert et al. (1999) Proc. SPIE-Int. Soc. Opt. Eng, 3618:413-417) or hydridosilsesquioxanes (see U.S. Pat. No. 6,087,064 to Lin et al.). These materials do not, however, have the requisite reactivity or solubility characteristics. The challenge in developing chemically amplified resists for 157 nm lithography lies in achieving suitable transparency in polymers that have acid-labile functionalities using industry standard developers in either exposed or unexposed areas, depending on whether the resist is positive or negative.
Homo- and copolymers of methyl α-trifluoromethylacrylate (MTFMA) and its derivatives have been found to be surprisingly transparent at 157 nm, with an optical density (OD) of less than 3/μm, whereas poly(methyl methacrylate) (PMMA) is highly absorbing, with an OD of 6/μm at 157 nm; see, for example, Ito et al. (2001), “Polymer Design for 157 nm Chemically Amplified Resists,” Proc. SPIE 4345: 273-284; Ito et al. (2001), “Novel Fluoropolymers for Use in 157 nm Lithography,” J. Photopolym. Sci. Technol. 14:583-593, and Chiba et al. (2000), “157 nm Resist Materials: a Progress Report,” J. Photopolym. Sci. Technol 13:657-664.
Unfortunately, MTFMA and its derivatives do not readily undergo radical homopolymerization, and polymers can be made only by anionic polymerization (see Ito et al. (1981), “Methyl α-Trifluoroacrylate, an E-Beam and UV Resist,” IBM Technical Disclosure Bulletin 24(2): 991). Although MTFMA-methacrylate copolymers are highly useful as 157 nm resist polymers, it is still desirable to identify comonomers that polymerize with α-trifluoromethylacrylic monomers by radical initiation. Radical polymerization is easy to run and economical they are an ideal process for preparation of resist polymers.
It has recently been discovered that MTFMA, α-trifluoromethylacrylic acid (TFMAA), and other esters conveniently undergo radical copolymerization with various vinyl ether derivatives. As discussed in Choi et al. (2000) “Design and Synthesis of New Photoresist Materials for ArF Lithography,” Proc. SPIE 3999:54-61, while vinyl ethers have been copolymerized with maleic anhydride (MA) for the design of 193 nm resists (VEMA), terpolymerization with a third functional monomer was required, because neither vinyl ether nor MA was functional. Also, the VEMA system has not proven useful for 157 nm lithography due to its excessive absorption.